1. Field of the Invention
The present invention relates to a surface acoustic wave filter comprising a plurality of SAW (Surface Acoustic Wave) resonators disposed on a piezoelectric substrate. More particularly, the present invention relates to a surface acoustic wave filter in which a plurality of SAW resonators define a ladder-type filter circuit.
2. Description of the Related Art
A conventional surface acoustic wave filter comprising a plurality of SAW resonators disposed on a piezoelectric substrate has been used as a high frequency band filter for a mobile communication device. Japanese Patent Publication (Kokoku) No. 56-19765 discloses a surface acoustic wave filter in which a plurality of SAW resonators define a ladder type circuit on a piezoelectric substrate.
In the above-mentioned surface acoustic wave filter, the resonance frequency fr of a series arm resonator, which forms a ladder-type circuit, is arranged to coincide with the anti-resonance frequency fa of a parallel arm resonator to which parallel arms are inserted so that the input/output impedance is equal to the characteristic impedance near the coincident frequency, thus forming a pass band. An example of the attenuation frequency characteristic of this type of surface acoustic wave filter is shown in FIG. 21.
Since the above-explained surface acoustic wave filter is used as a band pass filter, there has been a strong demand for increasing the attenuation amount in frequency regions other than the pass band. It is known that in order to increase the attenuation amount in the frequency region near the pass band, the electrostatic capacitance of the parallel arm resonator should preferably be increased relative to the electrostatic capacitance of the series arm resonator (Technical Report of the Institute of Electronics, Information and Communication Engineers, US95-25, pp.39-46, July 1995).
However, in the surface acoustic wave filter having a ladder-type circuit, if the capacitance ratio of the parallel arm resonator relative to the series arm resonator is varied, this causes an increase in insertion loss. More specifically, FIG. 22 shows the attenuation frequency characteristic when the ratio of the electrostatic capacitance of the parallel arm resonator to that of the series arm resonator is varied in the surface acoustic wave filter having a ladder-type circuit. In FIG. 22, the characteristics denoted as A to C are obtained by using surface acoustic wave filters A to C having a ladder-type circuit in which the parallel arm resonator has increasing amounts of capacitance in ascending order of A to C.
As is clear from the solid lines A to C, when the electrostatic capacitance of the parallel arm resonator is increased, the insertion loss within the pass band increases although the attenuation amount near the pass band increases.
That is, since the attenuation amount and the insertion loss in the frequency region near the pass band have a direct proportional relationship, it is impossible to increase only the attenuation amount in the frequency region near the pass band without increasing the insertion loss by merely changing the electrostatic capacitance ratio.
Accordingly, there has been a demand for a surface acoustic wave filter capable of increasing the attenuation amount in the frequency region near the pass band without increasing the insertion loss.